This invention relates to an improved apparatus and method for treating victims of cardiac arrest, and in particular for those patients who require a treatment regime consisting of cardiopulmonary resuscitation (CPR) and defibrillation electrotherapy.
A defibrillator delivers a high-voltage impulse to the heart in order to restore normal rhythm and contractile function in patients who are experiencing arrhythmia, such as ventricular fibrillation (“VF”) or ventricular tachycardia (“VT”) that is not accompanied by spontaneous circulation. There are several classes of defibrillators, including manual defibrillators and automated external defibrillators (“AEDs”). AEDs differ from manual defibrillators in that AEDs can automatically analyze the electrocardiogram (“ECG”) rhythm to decide if defibrillation is necessary. After deciding that a shock is needed, the AED arms itself for delivering an electrotherapeutic shock, and then the AED advises the user to press a shock button to deliver the defibrillation shock. An AED that operates in this manner is called semi-automatic. Fully automatic AEDs deliver the defibrillation shock without any user input. Fully automatic AEDs are generally called fully automatic defibrillators in order to reduce confusion in terminology.
FIG. 1 is an illustration of a defibrillator 1 being applied by a user 2 to resuscitate a patient 4 suffering from cardiac arrest. The defibrillator 1 may be in the form of an AED or a fully automatic defibrillator capable of being used by a first responder. The defibrillator 1 may also be in the form of a manual defibrillator for use by paramedics or other highly trained medical personnel. Two or more electrodes 6 are applied across the chest of the patient 4 by the user 2 in order to acquire an ECG signal from the patient's heart. The defibrillator 1 then analyzes the ECG signal for signs of arrhythmia with a shock analysis algorithm. Only if a shockable rhythm, such as VF or a non-perfusing ventricular tachycardia (VT), is detected does the defibrillator 1 arm itself to deliver a high voltage shock. The defibrillator 1 signals the user 2 via aural or visual prompts that a shock is advised. The user 2 then presses a shock button on the defibrillator 1 to deliver a defibrillation shock.
It is well established that the quicker that circulation can be restored (via CPR and defibrillation) after the onset of VF, the better the chances that the patient will survive the event. For this reason, many AEDs such as the one shown in FIG. 1 also incorporate a user interface including audible, aural, and visual prompting for guiding a user through a programmed sequence of CPR and defibrillation shocks. The user interface may include detailed aural prompting for properly applying CPR compressions, an audible metronome for guiding the user to the proper rate of compressions, a visual display to show the state and progress of the event, annunciators, flashing lights, and the like. The sequence is pre-programmed into the device in accordance with a protocol established by the local medical authority.
There are several ECG analysis algorithms which automatically analyze a patient's ECG to decide if a defibrillating shock is appropriate to treat the underlying cardiac rhythm. One such algorithm is generally described by Lyster et al. in the co-assigned U.S. Pat. No. 6,671,547 entitled “Adaptive analysis method for an electrotherapy device and apparatus” and herein incorporated by reference. The described algorithm relates to the Patient Analysis System (PAS) algorithm that is currently employed in AEDs, such as the Heartstart™ FR3 AED manufactured by Koninklijke Philips, N.V. of Andover, Mass.
But PAS and other existing ECG algorithms for determining a shockable condition require relatively noise-free ECG signals. All existing protocol sequences require the cessation of CPR during analysis because CPR causes artifact in the ECG which can mask VF when it is occurring, or can appear as VF when VF is not occurring. The former condition causes an undesirable reduction in sensitivity of the analysis, while the latter condition causes an undesirable reduction in specificity of the analysis. Consequently, all existing protocols of CPR and defibrillation require periodic “hands-off” periods of at least several seconds to allow the defibrillator to analyze the ECG with sufficient accuracy to be safe, useful, and effective to the patient.
Several problems arise from the need to interrupt CPR for ECG analysis. It has been shown that interruptions in CPR compressions, even for just a few seconds, may reduce the likelihood of a successful resuscitation. Thus, the required cessation of CPR for ECG analysis prior to delivering a defibrillating shock may reduce the chances of a successful patient outcome. And the delay in resuming CPR after defibrillation in order to assess the success of the shock may also impact the patient outcome.
Several prior art solutions to this problem have been developed, all directed toward reducing the amount of delay. One solution, for example, is to remove CPR noise artifact from the ECG signal by the use of adaptive filtering. Co-assigned U.S. Pat. No. 6,553,257 by Snyder et al. entitled “Interactive Method of Performing Cardiopulmonary Resuscitation with Minimal Delay to Defibrillation Shocks”, and herein incorporated by reference, describes such an adaptive filtering method.
Another alternative approach for analyzing ECG in the presence of CPR noise artifact involves wavelet transform analysis of ECG data streams. One example of this approach is described by Addison in U.S. Pat. No. 7,171,269 entitled “Method of Analysis of Medical Signals” and incorporated herein by reference. The '269 patent describes the use of wavelet transform analysis to decompose signals into heart and CPR-related signals. Another example of this approach is adopted by Coult et al. in International Patent Application No. PCT/US2012/045292 entitled “Systems and Methods for Analyzing Electrocardiograms to Detect Ventricular Fibrillation.” There, an electrocardiogram signal is interrogated by a wavelet, such as a Morlet, Myers, or Mexican Hat wavelet, prior to being analyzed and stratified into a shockable or non-shockable ECG.
Unfortunately, many of the ECG analyzing techniques lack the accuracy necessary to reliably determine a shockable rhythm in the presence of CPR noise artifact while avoiding “false positive” shock decisions. These techniques are also susceptible to external electrical noise, such as line noise, and have not been adopted.
For these reasons, other solutions have been developed to shorten the amount of “hands-off” ECG time needed to accurately determine a shockable rhythm. Co-assigned U.S. Pat. No. 7,463,922 by Snyder et al. entitled “Circuit and method for analyzing a patient's heart function using overlapping analysis windows”, also herein incorporated by reference, describes one such technique of using time-overlapped ECG data buffers to arrive at a quicker shock decision. Unfortunately, these prior art solutions serve only to reduce the delay time, but do not eliminate them entirely.
Another problem that arises from the existing inability to analyze ECG in the presence of artifact noise from CPR is that of refibrillation. A portion of patients that are successfully defibrillated, i.e. revert to an organized cardiac rhythm or asystole, subsequently re-enter VF several seconds to several minutes later. Some of these patients refibrillate during the fixed duration CPR period in which no ECG analysis is currently possible. Consequently, there is presently no treatment for addressing refibrillation except to wait for the protocol hands-off analysis period at the end of the CPR period. This delay in treating refibrillation is likely to be suboptimal for patient outcomes.
One-solution to the problem of refibrillation during CPR has been proposed, involving a measure of cardiac “vitality” during CPR. One such measure is the so-called “probability of Return of Spontaneous Circulation” (pROSC) score determined during CPR and described by Jorgenson et al. in U.S. patent application Ser. No. 13/881,380 entitled “Defibrillator with Dynamic Ongoing CPR Protocol”, incorporated herein by reference.
Another measure for predicting VF is the so-called Amplitude Spectrum Area (AMSA) Score described by Quan et al. in U.S. patent application Ser. No. 14/211,681 entitled “Treatment Guidance Based on Victim Circulatory Status and Prior Shock Outcome”. These approaches, however, only offer an indication of whether CPR should be discontinued to perform an ECG analysis for defibrillation purposes. Thus, additional delays can be induced by these solutions.